Medicinal patches usually leave behind a distinctive dirt rim, dirt ring or dark ring on the skin after being removed from the skin. This phenomenon becomes more important depending on the application duration and takes an especially problematic form, when the patch is worn for longer than 3 and up to 7 days. This situation especially occurs with an effective-ingredient transdermal patch, which is often applied for seven days during hormone substitution or hormonal contraception.
The dirt rim essentially forms by adherence of textile fibers, dirt and skin particles on the exposed cut edge of the adhesive layer. The adhesive layer can absorb particles, even by adhesive flow, according to the boundary surface energy and the adhesive properties. This is especially promoted by mechanical action on patches that are carried on the skin for a long time, in which adhesive material issues from the edge, which increases the adherence of particles at the dirty edge. The release of the patch starting from the edge after a long wearing time provides further surfaces on which dirt can be deposited.
A more or less large part of the dirt rim or ring remains on the skin after removal of the patch. It may be mechanically mostly poorly rubbed off and disappear after several days of the usual body care.
Of course improvements can be obtained by increasing the cohesion of the medicinal adhesive, for example by mixing in long-chain polymers or by chemical cross-linking. Frequently however the long time adherence is reduced with increased cohesion and reduced plasticity. Only a partially free-flowing adhesive with aggressive adhesive properties can guarantee adherence on the skin for up to one week. In this area of conflicting requirements for medicinal adhesives it is currently not possible to avoid formation of cosmetically interfering dirt rims or rings during long duration application.